In recent years, as the power supply for portable devices such as mobile phones and laptop computers, electric vehicles, and the like, nonaqueous electrolyte secondary batteries, such as lithium secondary batteries, have been attracting attention. Lithium secondary batteries have high energy density, low self-discharge rate, and excellent cycle performance. Nowadays, the mainstream lithium secondary batteries are small consumer batteries, mainly including 2-Ah or lower batteries for mobile phones. A large number of proposals have been made as positive active materials for lithium secondary batteries. The most commonly known material is a lithium-containing transition metal oxide having an operating voltage of about 4 V. The basic structure of a lithium-containing transition metal oxide is lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or lithium manganese oxide (LiMn2O4) with a spinel structure. In particular, a lithium cobalt oxide has excellent charge-discharge characteristics and energy density. Accordingly, a lithium cobalt oxide has been widely adopted as a positive active material for small-capacity lithium secondary batteries having a battery capacity up to 2 Ah.
However, in consideration of the future development of nonaqueous electrolyte batteries into medium-sized or large-sized batteries, especially into industrial batteries for which particularly high demands are expected, great importance is placed on safety. Accordingly, it cannot be said that the current specifications for small-sized batteries are necessarily sufficient. One of the factors thereof is the thermal instability of positive active materials. Various measures have been taken against thermal instability. However, no sufficient measures have been taken yet. Further, for industrial batteries, use in high-temperature environments, where small consumer batteries will not be used, should be assumed. In such a high-temperature environment, not only conventional nonaqueous electrolyte secondary batteries but also nickel-cadmium batteries or lead-acid batteries have extremely short life. Accordingly, under the present circumstances, none of conventional batteries satisfies the needs of users. Further, although a capacitor is the only one that is usable in such a temperature zone, capacitors do not satisfy the needs of users because of their low energy density. Therefore, there is a demand for a battery that lasts long life even in a high-temperature environment and has high energy density.
Recently, lithium iron phosphate (LiFePO4), a polyanionic positive active material having excellent thermal stability, has been attracting attention. In the polyanion moiety of LiFePO4, phosphorus and oxygen are linked together by a covalent bond. Accordingly, no oxygen is released even at high temperatures. Therefore, LiFePO4 shows high safety even when all Li is removed from the Li site. Accordingly, by using LiFePO4 as an active material for a battery, the safety of the battery can be dramatically improved. However, the operating potential of LiFePO4 is low (about 3.4 V). Accordingly, the energy density of LiFePO4 is lower than conventional 4-V class positive active materials. This low operating potential corresponds to the fact that a Fe2+/3+ redox reaction takes place near 3.4 V (vs. Li/Li+).
Meanwhile, a Mn2+/3+ redox reaction takes place near 4.1 V (vs. Li/Li+). Accordingly, studies have been made on lithium manganese phosphate (LiMnPO4) having Mn in place of Fe at the transition metal moiety of LiFePO4 in hope of obtaining an operating potential of about 4 V. However, as compared with LiFePO4, the electron conductivity of this material is extremely lower. Accordingly, there has been a problem in that discharge capacity itself is hardly obtained.
A reductive reaction in which lithium is electrochemically inserted into a lithium transition metal phosphate compound proceeds through a two-phase reaction. Accordingly, in LiFePO4, a plateau potential region occurs near 3.4 V (vs. Li/Li+) corresponding to the Fe2+/3+ redox potential. Then, in the case where a lithium transition metal phosphate compound contains a plurality of oxidizable and reducible elements as the transition metal, theoretically, a plurality of plateau potential regions appear each corresponding to the redox potential of each element. For example, in LiFeaMn1-aPO4, two stages are observed, i.e., a potential region near 3.4 V (vs. Li/Li+) corresponding to the Fe2+/3+ redox potential and a potential region near 4.1 V (vs. Li/Li+) corresponding to the Mn2+/3+ redox potential.
Patent Document 1 describes the charge-discharge curves of batteries using, as positive active materials, LiMn0.6Fe0.4PO4 (Example 1), LiMn0.7Fe0.3PO4 (Example 2), and LiMn0.75Fe0.25PO4 (Example 3). This document describes that in such a composition range, that is, in the composition range where the transition metal elements of a lithium transition metal phosphate compound are Mn and Fe, and the proportion of Mn in Mn and Fe is 0.6 to 0.75, a higher proportion of Mn results in a wider discharge region near 4 V corresponding to the Mn2+/3+ redox potential.
However, an increase in the proportion of Mn in LiFeaMn1-aPO4 reduces electron conductivity. Probably because of this, there has been a problem in that the entire discharging performance itself remarkably deteriorates.
In relation thereto, the invention defined in claim 1 of Patent Document 2 is “a positive active material to be contained in the positive electrode of a nonaqueous electrolyte battery, the positive active material containing a compound having an olivine structure represented by the general formula LiaMnbFecMdPO4, wherein M is one or more elements selected from Mg, Ti, V, Cr, Co, Ni, Cu, and Zn, and a, b, c, and d satisfy the relation 0<a<2, 0<b<0.8, 0<d<0.2, and b+c+d=1.” Further, this document describes, in paragraph 0027: “in the compound having an olivine structure represented by LiaMnbFecMdPO4, when the element ratio b of Mn in the formula is within a range of more than 0 and less than 0.8, the battery 1 can be provided with higher battery capacity. Further, in this case, electron conductivity increases. Accordingly, the positive electrode 2 can be provided with improved electrical conductivity.” However, this document describes, in paragraph 0026: “when the element ratio of Mn is 0.8 or more, the content of Mn that reduces electrical conductivity is too high. Accordingly, even when a part of Mn and/or a part of Fe is substituted with a predetermined element, it is difficult to increase electron conductivity.” Meanwhile, in the present invention, it has been found that by applying Ni within a specific range to a material in which the element ratio of Mn is 0.9, the capacity is remarkably improved. Therefore, the present invention cannot be readily derived from the descriptions of Patent Document 2.
Further, the invention defined in claim 1 of Patent Document 3 is “a method for producing a positive electrode active material, including the step of mixing a metal-doped lithium manganese phosphate LiMn1-xMxPO4 (wherein 0<x≦0.1, and M represents a doping metal element) with a carbon source, and heat-treating the resulting mixture in an inert gas atmosphere.” An object of the invention is “to provide a production method that enables the easy mass-production of a positive active material having rate characteristics suitable for nonaqueous electrolyte secondary batteries, and also to provide a high-performance nonaqueous electrolyte battery having a positive active material obtained by the method.” Further, this document describes, in paragraph 0016: “in the above general formula: LiMn1-xMxPO4, the doping metal element M other than Mn in the compound is not particularly limited, but is preferably at least one selected from Co, Ni, Fe, Mg, Zn, and Cu. The x representing the proportion of the metal element M other than Mn is 0<x≦0.1, preferably 0.003≦x≦0.05, more preferably 0.005≦x≦0.05, more preferably 0.007≦x≦0.03, and particularly 0.01≦x≦0.03. The positive active material of the invention is characterized by the use of a metal-doped lithium manganese phosphate having a doping metal in an extremely small proportion.” Thus, in this document, as candidates for the metal element M, Co, Ni, Fe, Mg, Zn, and Cu are listed equally without any distinction.
However, Patent Document 3 merely describes, in Examples, specific cases of substitution with Mg alone (0.01, 0.05, 0.10) and substitution with Ti alone (0.01, 0.05, 0.10). Therefore, from the descriptions of Patent Document 3, it cannot be readily derived that in the case where Mn is 0.9 and the substituting elements are Fe and Ni, the proportion of Ni in Fe and Ni needs to be 30% or more and 90% or less.    [Patent Document 1] JP-A-2001-307732    [Patent Document 2] JP-A-2004-63422    [Patent Document 3] JP-A-2008-130525